In X-ray CT systems, X-rays are used to image internal structures and features of a region of a subject or an object. The terms “subject” and “object” shall include anything capable of being imaged. The imaging is performed by an X-ray CT system, which images internal structures and features of a plurality of thin planar slices or a 3D volume of a region of an object using X-rays. For medical applications, the imaging objects include human bodies.
An X-ray CT system generally comprises an X-ray source that provides a cone-shaped X-ray beam and an array of closely spaced X-ray detectors that face the X-ray source. The X-ray source and the array of detectors are mounted in a gantry so that a patient being imaged with the CT system, generally lying on an appropriate support couch, can be positioned within the gantry between the X-ray source and the array of detectors. The gantry and the couch are moveable relative to each other so that the X-ray source and the detector array can be positioned axially at desired locations along the patient's body.
The gantry comprises a stationary structure referred to as a stator and a rotary element referred to as a rotor, which is mounted to the stator so that the rotor is rotatable about the axial direction. In third generation CT systems, the X-ray source and the array of detectors are mounted on the rotor. Angular positions of the rotor about the axial direction are controllable so that the X-ray source can be positioned at desired angles, referred to as view angles, around a patient's body.
To image a slice in a region of a patient's body, the X-ray source is positioned at an axial position of the slice and the X-ray source is rotated around the slice to illuminate the slice with X-rays from a plurality of different view angles. At each view angle, detectors in the array of detectors generate signals responsive to the intensity of X-rays from the source that passes through the slice. The signals are processed to determine the amounts, by which X-rays from the X-ray source are attenuated over various path lengths through the slice that the X-rays traverse, in passing though the slice from the X-ray source to the detectors. The amounts, by which the X-rays are attenuated, are used to determine the X-ray absorption coefficients of materials in the slice as a function of position in the slice. The absorption coefficients are used to generate an image of the slice and to identify compositions and densities of tissues in the slice.
The X-ray detectors comprised in a detector array of CT system are generally packaged in a plurality of modules, hereinafter referred to as detector modules, each of which comprises a plurality of X-ray detector elements. Most modern CT systems are multi-slice CT systems designed to simultaneously image a plurality of slices of a patient. The X-ray detector elements in each detector module of a multi-slice CT scanner are arranged in a matrix of rows and columns. The X-ray detector matrices of any two CT detector modules in a CT system are substantially identical and comprise a same number of rows of detector elements and a same number of columns of detector elements. The modules are positioned one adjacent to and contiguous with the other in a closely packed array with their rows of detectors aligned end to end so that the X-ray detector elements form a plurality of long parallel rows of X-ray detector elements.
A multi-slice X-ray CT system is usually named or featured by the maximum number of slices that it can simultaneously image, for example, an 8-slice CT system means that it can simultaneously image at most 8 slices; a 16-slice CT system can simultaneously image at most 16 slices.
The X-ray detector elements in each long row of the detector array lie on an arc of a circle having its center located at a focal point of the CT system's X-ray source, and the design of these detector elements and the detector modules is specifically determined by the radius of the circle, which is hereinafter referred to as focusing distance. The design of X-ray detector modules placed on the arc of one focusing distance of one CT system cannot therefore be used on another CT system of a different focusing distance.
Each detector element in a scintillator array is comprised of a finite active area for detecting X-ray photons and generating second-energy photons or electric charges when using X-ray direct conversion materials. Detector elements are surrounded by non-active areas, referred hereafter as gaps, which do not generate responses to X-ray photons.
An X-ray detector array typically includes an anti-scatter collimator having a plurality of anti-scatter plates for collimating X-ray beams received at each detector element: a scintillator for converting X-rays to light energy adjacent to the collimator, and a photodiode for receiving the light energy from the coupled scintillator and producing electric charges therefrom.
The electric charges generated from the detector array are then fed into Analog to Digital Conversion (ADC) Integrated Chips (ICs) to generate digital signals for processing and reconstructing tomographic images of scanned objects.